In the case of carrying out the laser cutting processing on a plate shaped workpiece such as a mild steel plate, the piercing processing is carried out on the above noted workpiece and the laser cutting processing on the workpiece is started from this piercing processed position. The above noted piercing processing is carried out almost instantaneously (several hundreds msec) in the case where the workpiece is a thin plate, so that there are not much of problems even in the case of the laser cutting processing in which the piercing processing is repeated for a plurality of times. However, when the workpiece is a thick plate, the piercing processing will require some time. For example, when the piercing processing using a 4 kW carbon dioxide gas laser oscillator is carried out on a mild steel plate with a plate thickness of 19 mm, it is going to require about 20 seconds or so for the piercing processing time. Therefore, in the case of carrying out the laser cutting processing on the workpiece, there is a desire to shorten said piercing processing time. Note that documents that are considered as relevant to the present invention include Patent Document 1.
The piercing processing method as described in said Patent Document 1 is such that a nitrogen gas is used as an assist gas and the piercing processing is carried out at high output at an initial stage of the piercing processing, and then an oxygen gas is used as an assist gas and the piercing processing is carried out at low output. Therefore, at the initial stage of the piercing processing, an oxidization reaction is going to be suppressed as a nitrogen layer is formed at a piercing processed hole. Consequently, it is going to be such that an excessive burning can be prevented and the piercing processing can be carried out in shorter time, even in the case where the laser output is made larger.
However, in the laser processing machine mounted with a fiber laser oscillator, even when the piercing processing method as described in said Patent Document 1 is practiced, it has not been possible to shorten the piercing processing time significantly. Namely, in the laser processing machine equipped with a fiber laser oscillator, there are cases where an axicon lens is used for a condensing lenmildhe laser processing head, in order to carry out the laser cutting processing on a mild steel plate as a workpiece appropriately. As this axicon lens forms ring shaped beams from incident laser beams, there has been a possibility that the energy of the laser beams is not being able to act on the workpiece effectively at a time of the piercing processing on the workpiece.